The invention relates to a method and a pump apparatus for the generation of an adjustable, substantially constant volume flow of a fluid and to the use of this method.
The necessity exists of transporting a fluid in a metered manner in many industrial processes and in the conduct of numerous methods; i.e. a substantially constant volume flow of the fluid should be forwarded in the sense that the quantity of fluid forwarded per unit of time is constant. An example which can be given here is chemical-mechanical polishing (CMP) processes such as are used in the semi-conductor industry. In such processes, a suspension, which is usually called a slurry and is typically made up of very fine solid particles and a liquid, is brought onto a rotating wafer and serves there for the polishing or lapping of the very fine semi-conductor structures. In this respect, it is necessary for the suspension (slurry) to be applied to the wafer surface with a constant, adjustable volume flow, i.e. in a metered manner.
In other processes, for example the application of photosensitive resist onto the wafer, it is necessary to transport a metered quantity of fluid, which can as a rule be realized by the transportation of a constant volume flow over a predetermined time.
Volumetric pumps such as peristaltic pumps, gear pumps, piston pumps and diaphragm pumps are used nowadays for such metered forwarding. Such volumetric pumps have the characteristic that they forward a well-defined quantity of fluid per working cycle. With a piston pump, for example, the fluid volume forwarded per stroke is fixed by the piston area and the piston stroke. A constant volume flow can thus be generated because the same volume of fluid is forwarded with each stroke of the piston.
Volumetric pumps, however, have characteristics which can prove to be disadvantageous in some applications, for example they generally generate pulsating fluid flows. The pressure fluctuations associated with this can prove to be very disturbing. They can also facilitate deposits or agglomeration of particles. Moreover, with special liquids, such as with the already mentioned slurry suspensions, the solid particles can result in substantial damage to the pump due to abrasion, in particular to the sealing components between the suction side and the pressure side. For instance, leaks can occur due to the abrasive particles, for example, along the piston in a piston pump, as the piston rings and/or the cylinder wall are damaged.
It is an object of the invention to provide another method and an apparatus with which an adjustable, substantially constant volume flow of a fluid can be generated. The method should be capable of being realized as simply as possible apparatus-wise and also, in particular, be suitable for the transportation of suspensions, for example slurry.
In accordance with the invention, a method is provided for the generation of an adjustable, substantially constant volume flow of a fluid by means of a pump apparatus, in which method the fluid is forwarded by the pump apparatus from a reservoir into a flow connection. A rotary pump is used as the pump apparatus and the rotary pump is operated at an efficiency which is less than half of the maximum efficiency of the rotary pump. The rotary pump is preferably operated at at most twenty percent of its maximum efficiency.
Of importance for the invention is the recognition that with such operating states of the rotary pump, which correspond to a very low efficiency, there is a unique relationship between the speed of rotation of the pump, more specifically the speed of rotation of the pump rotor, and the volume flow forwarded by the pump. A constant volume flow of the fluid can thus be set in this operating range directly via the speed of rotation of the pump rotor without sensors being necessary for the detection of the volume flow or control circuits for the rotary pump. For this reason, the method of the invention can be realized very easily apparatus-wise. The desired volume flow is simply controlled and adjusted via the speed of rotation of the rotary pump.
The volume flow generated by the rotary pump is free of pulsations as a result of the operating principle of a rotary pump, whereby the risk of unwanted deposits is also substantially reduced.
Furthermore, with a rotary pump, the inlet is in constant flow communication with the outlet of the pump. There are no diaphragms, pistons or similar components which can seal off the outlet with respect to the inlet. This reduces the risk of damage to the pump when forwarding abrasive particles.
Moreover, a rotary pump can also work with a closed outlet, that is, with a forwarded volume flow of zero, without incurring damage. The possibility thereby exists to restrict the constant volume flow time-wise or to interrupt it by an ON/OFF valve downstream of the pump. Consequently, with the method of the invention, the metered dispensing of a predetermined quantity of fluid is also possible without complex control mechanisms being required for the forwarding performance of the pump.
It is particularly advantageous if the fluid is forwarded to a dispensing apparatus through the flow connection, with the total flow resistance of the flow connection and of the dispensing apparatus being selected such that its mean value is substantially larger than the fluctuations of the flow resistance during operationxe2x80x94that is, at least twice as large. Then, namely, the flow resistance of the dispensing apparatus (load resistance) can, for example, vary without the flowxe2x80x94that is, the volume flow forwarded by the pumpxe2x80x94changing.
The method of operating a rotary pump with an efficiency which is less than half the maximum efficiency of the rotary pump can also be advantageously used to determine the viscosity, and in particular the dynamic viscosity, of a fluid. This application of the rotary pump is independent of whether the pump is used for the generation of a constant volume flow or not.
The method of the invention can preferably be used for the forwarding of suspensions, in particular for the forwarding of slurry, especially in a CMP process.
The pump apparatus of the invention is characterized in that it is designed as a rotary pump.